The preset invention relates to a lock-up clutch controller.
A vehicle such as an automobile described in Japanese Laid-Open Patent Publication No. 2006-125629 includes a torque converter, a lock-up clutch, and a controller. The torque converter transmits power between the engine and the transmission through fluid. The lock-up clutch is capable of engaging the engine side components of the torque converter directly with the transmission side components of the torque converter. The controller controls operation of the lock-up clutch. To control the operation of the lock-up clutch, the controller switches the lock-up clutch to one of a directly engaged state, a disengaged state, and a slip state. The slip state of the lock-up clutch includes a deceleration slip state and an acceleration slip state. Specifically, the deceleration slip state of the lock-up clutch is for when the depression amount of the accelerator pedal (the accelerator operating amount) is 0 and the acceleration slip state of the lock-up clutch is for when the accelerator operating amount is greater than 0.
In control of the operation of the lock-up clutch, a direct engagement range, a disengagement range, an acceleration slip range, and a deceleration slip range are set in correspondence with the accelerator operating amount and the vehicle speed. In the direct engagement range, the lock-up clutch is in the directly engaged state. In the disengagement range, the lock-up clutch is in the disengaged state. In the deceleration and acceleration slip states, the lock-up clutch is in the slip state. The lock-up clutch is switched to one of the directly engaged state, the disengaged state, and the slip state depending on which of these ranges a condition value that is determined by the current accelerator operating amount and vehicle speed falls in. The direct engagement range, the disengagement range, the acceleration slip range, and the deceleration slip range are set, for example, in the manners described below. Specifically, the direct engagement range is set for a high vehicle speed range. The disengagement range is set for a low vehicle speed range. The acceleration slip range is set between the direct engagement range and the disengagement range. The deceleration slip range is the range corresponding to the accelerator operating amount 0 and set adjacent to the acceleration slip range and the direct engagement range on the decreasing side of the accelerator operating amount.
Specifically, the direct engagement range and the disengagement range are set for the high vehicle speed range and the low vehicle speed range, respectively, for the purposes described below. One of the purposes is to reduce the fuel consumption of the engine by maintaining the lock-up clutch in the directly engaged state in the broadest possible range to improve the power transmission efficiency. The other purpose is to suppress muffled noise of the engine generated by switching the lock-up clutch to the directly engaged state when the vehicle speed is low. The acceleration slip range is set between the direct engagement range and the disengagement range in order to further reduce the fuel consumption of the engine. Specifically, the power transmission efficiency from the engine to the transmission is maximally enhanced by maintaining the lock-up clutch in the slip state, which is closest possible to the directly engaged state, and enlarging the range of the slip state to cover a range of low vehicle speed with respect to the direct engagement range. The deceleration slip range corresponds to the accelerator operating amount 0 and is set adjacent to the acceleration slip range and the direct engagement range and on the decreasing side of the acceleration operating amount in order to maximize the fuel saving effect through the engine fuel cut-off.
The engine fuel cut-off is carried out on condition that the accelerator operating amount is 0 and the engine speed is greater than or equal to a predetermined value. The predetermined value is greater than a target idle speed. Since autonomous operation of the engine is suspended when the fuel cut-off is executed, the engine speed drops to a value less than the predetermined value, which may stop the fuel cut-off. To solve this problem, the lock-up clutch is held in the slip state to ensure power transmission from the wheels to the engine, thus maintaining the engine speed at a value greater than or equal to the predetermined value as long as possible, so that the fuel cut-off can be continued. In order to maximize the saving effect through the fuel cut-off of the engine, the boundary of the deceleration slip range on the lower vehicle speed side is set to correspond to the lowest possible vehicle speed. In this manner, the deceleration slip range is set adjacent to the acceleration slip range and the direct engagement range in a range where the accelerator operating amount is smaller than those of the acceleration slip range and the direct engagement range.
Control of the operation of the lock-up clutch in the acceleration slip range and the proximity of this range will now be explained in detail.
When the condition value determined by the current accelerator operating amount and the current vehicle speed falls in the acceleration slip range, an acceleration slip executing command is generated. In response to the acceleration slip executing command, the lock-up clutch is operated to switch to the acceleration slip state. Accordingly, when the current condition value is in the disengagement range and then moves to the acceleration slip state as the vehicle speed increases, the acceleration slip executing command is generated to operate the lock-up clutch to switch to the acceleration slip state. If the current condition value is outside the acceleration slip range, an acceleration slip stopping command is generated. In response to the acceleration slip stopping command, the lock-up clutch is operated in accordance with the range in which the current condition value exists. Accordingly, for example, when the current condition value is in the acceleration slip range and then moves to the disengagement range as the vehicle speed decreases, the acceleration slip stopping command is generated and the lock-up clutch is operated to switch to the disengaged state.
If the condition value changes frequently between the disengagement range and the acceleration slip range, the activation state of the lock-up clutch changes frequently between the slip state and the disengaged state. This may adversely influence the lock-up clutch. To suppress such frequent changes of the actuation state of the lock-up clutch, a hysteresis range is set in a section of the acceleration slip range corresponding to a relatively low vehicle speed range. If the current condition value is in the hysteresis range, the command regarding the acceleration slip state that was generated immediately before the condition value has entered the hysteresis range is maintained.
In other words, if the current condition value enters the hysteresis range while moving from the section of the acceleration slip range other than the hysteresis range to the disengagement range, the acceleration slip executing command is maintained as long as the condition value is in the hysteresis range. As a result, the actuation state of the lock-up clutch is maintained as the acceleration slip state. If the current condition value enters the hysteresis range while changing from the disengagement range to a range of the acceleration slip range other than the hysteresis range, the acceleration slip stopping command is maintained as long as the condition value is in the hysteresis range. This maintains the lock-up clutch in the disengaged state. In other words, the lock-up clutch is maintained in the disengaged state if the current condition value is in the hysteresis range and the acceleration slip stopping command is generated.
The above-described disadvantage is suppressed by setting the hysteresis range and controlling the actuation state of the lock-up clutch between the acceleration slip state and the disengaged state in correspondence with the acceleration slip executing command or the acceleration slip stopping command, which is maintained when the current condition value enters the hysteresis range. In other words, even when the condition value frequently changes between the disengagement range and the acceleration slip range (the hysteresis range), the lock-up clutch is prevented from changing frequently between the acceleration slip state and the disengaged state. This suppresses the disadvantageous influence of frequent change of the actuation state of the lock-up clutch on the lock-up clutch.
However, with regard to setting of the hysteresis range and the commands regarding the acceleration slip state, the problem described below may occur.
When the current condition value is in the hysteresis range and the acceleration slip execution command has been made, the activation state of the lock-up clutch is maintained in the disengaged state if the accelerator pedal is operated to decrease the accelerator operating amount to 0 and then immediately increase the accelerator operating amount to a value greater than 0, and the condition value is maintained in the hysteresis range thereafter. This increases the fuel consumption of the engine. Specifically, when the accelerator operating amount decreases to 0, the acceleration slip stopping command is generated and the condition value moves from the hysteresis range to the deceleration slip range. Afterwards, the accelerator operating amount becomes a value greater than 0 and the condition value returns from the deceleration slip range to the hysteresis range. In this state, the current condition value is held in the hysteresis range and the acceleration slip stopping command is maintained. That is, when the condition value is in the hysteresis range and the acceleration slip stopping command is generated, the lock-up clutch is in the disengaged state. The lock-up clutch is maintained in the disengaged state as long as the condition value remains in the hysteresis range and the acceleration slip stopping command is maintained, thus lowering the power transmission efficiency between the engine and the transmission. This deteriorates the fuel economy of the engine.